A study is being made on the application of a nitride semiconductor to high-voltage resistance high-output semiconductor devices by taking advantage of the characteristic features of the nitride semiconductor, such as high saturated electron velocity, a wide bandgap and the like. For example, the bandgap of GaN which is a nitride semiconductor is 3.4 eV, higher than the bandgap (1.1 eV) of and Si and the bandgap (1.4 eV) of GaAs, thus having high breakdown field strength. Accordingly, GaN holds great promise as a material of a semiconductor device for power supplies from which high-voltage operation and high output are available.
Many reports have been made of a field-effect transistor, a high electron mobility transistor (HEMT) in particular, as a semiconductor device using a nitride semiconductor. For example, an AlGaN/GaN HEMT using GaN as an electron transit layer and AlGaN as an electron supply layer is a focus of attention as a GaN-based HEMT (GaN-HEMT). In the AlGaN/GaN HEMT, strain due to a difference in lattice constant between GaN and AlGaN arises in AlGaN. Consequently, there is obtained a high-concentration two-dimensional electron gas (2DEG) due to piezoelectric polarization and the spontaneous polarization of AlGaN caused by the strain. Accordingly, the HEMT is expected for use as a high-efficiency switch element or a high-voltage resistance power device for electric vehicles and the like.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2010-153493    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2009-49288    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2008-71988
However, the GaN-HEMT has the disadvantage of generally lacking avalanche resistance and being significantly vulnerable to surges. In addition, unlike Si-based semiconductor devices, the GaN-HEMT does not have a body diode. Consequently, a diode needs to be connected externally as a so-called freewheel diode (FWD), in order to apply the GaN-HEMT to, for example, an inverter circuit (full-bridge inverter circuit).
In the GaN-HEMT, a parasitic capacitance Cgs between a gate electrode and a source electrode and a parasitic capacitance Cgd between the gate electrode and a drain electrode may increase due to the concentration of 2DEG being high. Hence, in order to reduce the parasitic capacitances Cgs and Cgd, a field plate using a metal material is formed in some cases on a gate insulation film, so as to be adjacent to the gate electrode on the drain electrode side. In this case, however, the parasitic capacitances Cds and Cgd are large since a depletion layer has difficulty in extending under the field plate. Thus, there is concern that the large capacitances may degrade the speed of device operation. When the field plate is provided, voltage resistance is secured by an insulation film (a gate insulation film, for example) interposed between the field plate and a compound semiconductor layer. This causes another problem that electric fields center on the insulation film, and thus voltage resistance fails to be improved. It is also viewed as a problem that electron holes produced by impact ionization or the like permanently exist in a device.